Disbond resistant composite stiffener runout

ABSTRACT

The disclosure herein provides for disbond resistant stringer runouts. Various aspects may be used independently or in combination to mitigate disbonding associated with the stiffener runout under operational loads. Aspects include a stiffener runout having rounded base flange corners. A recessed notch may be incorporated within the base flange to provide a flange termination point that is forward of the web termination point, allowing the web to terminate prior to the base flange. The web may be trimmed from a full height to a reduced height at the web termination point. The base flange may be co-bonded to the underlying composite structure via a scarf joint. Perimeter clamp radius fillers may be used to concentrate a clamping force around the perimeter of the base flange.

BACKGROUND

Stringers are structural components that are often used to stiffenpanels and other structures. With composite panels, composite stringersmay be co-bonded, or secondarily bonded, onto the panel to prevent thepanel from buckling or otherwise failing when subjected to compressiveloads. The combined panel and stringers may then be secured to asubstructure.

While stiffeners provide adequate reinforcement of the correspondingcomposite panel with respect to compressive loads, the stiffenerrunouts, or areas at which the stringers terminate, may experiencedisbonding between the stringers and the panel under certain conditions.The sudden discontinuity of the load path, combined with a low compositeinterlaminar toughness, may result in a structural component that maydelaminate or disbond at operational loads.

Traditionally, the disbond potential that exists at the stiffener runoutlocations of a composite structure has been accommodated via varioustechniques. For example, the architecture of a composite panel may bedesigned to move the stiffener terminations to an edge of the panel.However, doing so may negatively affect the cost and weight of thecorresponding panel. According to an alternative traditional solution,the disbond may be allowed to occur at operational loads and fastenersused to arrest the disbond. With this solution, extensive certificationtesting is required and any amount of disbond, even though non-critical,may be disconcerting to a customer.

Another conventional method for controlling disbond at stiffener runoutlocations includes the use of flat metallic radius fillers secured tothe stiffeners at the termination ends. However, these conventionalradius fillers are not efficient at arresting the disbond under theflange, and provide no arrest capabilities under a web of a stringer.Relatively large mechanically attached metallic fittings may also beused to transfer loads from stringers into the attached compositestructure. A disadvantage of these types of fittings is that thefittings increase the weight of the corresponding structure and may beexpensive to install, while not completely suppressing the disbond atoperational loads.

It is with respect to these considerations and others that thedisclosure made herein is presented.

SUMMARY

It should be appreciated that this Summary is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This Summary is not intended to beused to limit the scope of the claimed subject matter.

Apparatus and methods described herein provide for disbond resistantstiffener runouts. According to one aspect, a stiffener runout mayinclude a web and a base flange. The web may be configured in a verticalorientation having a front surface, a rear surface, a bottom edge, andweb termination point that is positioned at an outermost location of theweb. The base flange may include two flanges extending horizontally fromthe front and rear surfaces of the web, respectively. The base flangemay include a flange termination point that is positioned at anoutermost location of the base flange, which is forward of the webtermination point.

According to another aspect, a method of controlling bondline separationbetween a composite stiffener and a composite structure may be provided.The method may include providing a composite stiffener having a web anda base flange. The web may terminate at a web termination point, whilethe base flange terminates at a flange termination point that is forwardof the web termination point. The base flange may include roundedcorners. The web may be trimmed from a full height prior to the webtermination point to a reduced height at the web termination point. Arecessed notch may be provided within the base flange at the flangetermination point, with the web termination point being positioned atthe center of the recessed notch. The base flange may be bonded to thecomposite structure.

According to yet another aspect, a stiffener runout assembly may includea stiffener runout, a composite structure bonded to the stiffenerrunout, a pair of perimeter clamp radius fillers, and a number offasteners. The stiffener runout may include a web trimmed from a fullheight to a reduced height at the web termination point at an outermostlocation of the web. The stiffener runout may also include the baseflange that extends horizontally in opposite directions from the web andincludes a flange termination point at an outermost location of the baseflange. The flange termination point may be positioned forward of theweb termination point, with the web termination point positioned withina recessed notch of the base flange.

The composite structure may be bonded to the base flange of thestiffener runout at a scarf joint. The pair of perimeter clamp radiusfillers may be positioned on a top surface of the base flange onopposing sides of the web. Each perimeter clamp radius filler mayinclude a bottom surface that is configured to contact the top surfaceof the base flange around the perimeter of the top surface to create acontact area between the perimeter clamp radius filler and the perimeterof the top surface. The contact area may define a relief cavity betweenthe perimeter clamp radius filler and the top surface of the baseflange. The fasteners may secure the perimeter clamp radius fillers tothe top surface of the base flange, extending through a top surface ofthe perimeter clamp radius fillers, through the relief cavities, throughthe base flange, and through the composite structure.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments, further details of whichcan be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a stiffener runout assembly illustratingaspects of a stiffener runout for controlling disbond according tovarious embodiments presented herein;

FIG. 2 is a top view of a stiffener runout illustrating aspects of thestiffener runout for controlling disbond according to variousembodiments presented herein;

FIG. 3 is a side view of a stiffener runout assembly illustratingaspects of a stiffener runout for controlling disbond according tovarious embodiments presented herein;

FIG. 4 is a side view of a stiffener runout assembly illustratingaspects of a stiffener runout and scarf joint for controlling disbondaccording to various embodiments presented herein;

FIG. 5 is a side view of a stiffener runout assembly illustratingaspects of a stiffener runout and alternative scarf joint forcontrolling disbond according to various embodiments presented herein;and

FIG. 6 is a flow diagram showing a method of controlling bondlineseparation between a composite stiffener and a composite structureaccording to various embodiments presented herein.

DETAILED DESCRIPTION

The following detailed description is directed to controlling bondlineseparation between a composite stiffener and a composite structure. Asdiscussed above, delamination or disbond may occur between stringers andco-bonded composite structures at stringer runout locations whensubjected to operational loads. As composite materials are increasinglyused in the construction of vehicles such as aircraft, controllingdelamination and disbonding at the stringer runout locations is aprimary concern. Solutions to the stringer disbonding issue shouldcontemplate weight factors, as well as cost and complexity ofmanufacturing, particularly within the aircraft industry.

Utilizing the concepts and technologies described herein, stringerrunouts may incorporate various stringer runout aspects, alone or incombination, to prevent disbonding between composite stringers andco-bonded composite structures while minimizing weight and manufacturingcomplexity and cost. The various embodiments described below will bedescribed within the context of a composite aircraft stringer co-bondedwith composite aircraft skin. It should be appreciated that the variousembodiments are not limited to these specific components, or to usedwithin aircraft. Rather, the concepts described below may equally applyto any implementation in which a stringer or similar stiffeningcomponent is bonded to a composite structure.

In the following detailed description, references are made to theaccompanying drawings that form a part hereof, and which are shown byway of illustration, specific embodiments, or examples. Referring now tothe drawings, in which like numerals represent like elements through theseveral figures, a disbond resistant stiffener runout and assembly, aswell as methods form controlling disbond between a composite stiffenerand a composite structure, will be described.

FIG. 1 shows a front view of a stiffener runout assembly 100. Accordingto this illustrative embodiment, the stiffener runout assembly 100includes a stiffener runout 102 co-bonded onto a composite structure104. The composite structure 104 may be any composite panel, sheet, orother component for which stiffening would be desirable, such as anaircraft skin. The stiffener runout 102 includes a web 106, a baseflange 108 that includes a first flange portion 108A and a second flangeportion 108B, and a cap 110. A noodle 114 is bonded within the regioncreated between the web 106 and the first and second flange portions108A and 108B.

As shown in FIG. 1, the web may extend vertically in relation to thecomposite structure 104. The base flange 108 may be positioned at abottom edge of the web 106, with the first flange portion 108A extendingsubstantially horizontally from a front surface 120 of the web 106 andthe second flange portion 108B extending substantially horizontally froma rear surface 122 of the web 106. The base flange 108 is bonded to thecomposite structure 104. According to various embodiments, the stiffenerrunout assembly 100 may include perimeter clamp radius fillers 112positioned on the first flange portions 108A and 108B to assist withpreventing disbond at the forward edges of the stiffener runout 102. Thevarious features of the perimeter clamp radius fillers 112 will bedescribed in greater detail below with respect to FIGS. 2 and 3. Itshould be appreciated that the stiffener runout 102 may includeadditional features and/or varying proportions or configurations ascompared to the stiffener runout 102 shown in the figures withoutdeparting from the scope of this disclosure. The various figures may notbe drawn to scale and are shown for illustrated purposes only.

FIGS. 2 and 3 show top and side views, respectively, of a stiffenerrunout 102 according to one embodiment. As described above, thestiffener runout 102 may include a web 106 and a base flange 108. Thereare multiple features of the stiffener runout 102 that independently andcollectively mitigate or eliminate any disbond between the stiffenerrunout 102 and the composite structure 104. One such feature includesthe rounded corners 202 of the base flange 108. Conventional stiffenersinclude base flanges that have squared off corners. The square cornersare easier to manufacture. Historically, delamination tends to occur atthe corners of the base flange first, particularly when a partial shearloading exists. However, by trimming or otherwise creating the roundedcorners 202 as shown in FIG. 2, the loads through the corners of thebase flange 108 are more easily distributed, reducing the potential fordelamination at those locations.

Another feature disclosed herein to control disbonding includes therecessed notch 204 in base flange 108. According to various embodiments,the base flange 108 terminates at a flange termination point 206. Theflange termination point 206 may be the most forward location of thestiffener runout 102. Similarly, the web 106 terminates at a webtermination point 208. Conventional stiffeners include webs and baseflanges that terminate at identical locations. The forward edge of astringer runout is typically stiffer under the web 106 and noodle 114 ascompared to the base flange 108. This stiffer central location of thetypical stringer forward edge allows for disbonds to occur at thislocation before occurring at other locations of the base flange 108.

However, as shown clearly in FIG. 2, the stiffener runout 102 of thevarious embodiments disclosed herein includes a flange termination point206 that is forward of the web termination point 208. The webtermination point 208 is located within a recessed notch 204, which maybe on a semicircular or arcuate edge of the base flange 108 aft of theflange termination point 206. In doing so, any curvature between thetermination end of the stiffener runout 102 and the composite structure104 may be matched, which mitigates the disbonding under the web 106 andnoodle 114.

Similarly, as seen in FIG. 3, the web 106 may be trimmed from a fullheight 304 to a reduced height 306 at the web termination point 208.According to one embodiment, the reduced height 306 may be substantiallyflush with a top surface of the base flange 108. By tapering or reducingthe height of the web 106, the loads within the stringer 102 are reducedat the flange termination point 206, which reduces the potential fordisbond. Additionally, reducing the height of the web 106 reduces thestiffness of the stiffener runout 102 along the central axis of thestiffener, which similar to the benefits described above with respect tothe recessed notch 204, mitigates the disbonding under the web 106 andnoodle 114. According to one embodiment, the leading edge 302 of the web106 is tapered according to a curved configuration as shown in FIG. 3.

An additional feature used to control disbonding includes the use of theperimeter clamp radius fillers 112, as shown in FIGS. 2 and 3. Asdiscussed above, traditional radius fillers are flat metallic componentsthat may be secured to a base flange in order to provide pressure to theflange and corresponding bond between the flange and the underlyingcomposite structure in an effort to mitigate disbonding. However,traditional radius fillers do not efficiently handle panel deformation.For example, when a composite structure 104 or panel deforms under loadsuch that the panel under a portion of the traditional radius fillerflexes downward, the traditional radius filler will tend to lift off ofthe surface, either over the portion of the panel that is deflectingdownward, or as that portion of the radius filler deflects downward withthe panel, an opposing edge of the traditional radius filler may liftoff of the surface.

The perimeter clamp radius fillers 112 described herein alleviate thesedeficiencies of traditional radius fillers by concentrating the clampingforce around the perimeter of the base flange 108 where the disbondingis most likely to occur. Moreover, a relief cavity 212 within a centralportion of the perimeter clamp radius filler 112 accommodates anydeformation of the composite structure 104 and the corresponding baseflange 108. The perimeter clamp radius fillers 112 may be metallic, orany other suitable material.

According to one embodiment, each perimeter clamp radius filler 112includes a bottom surface that projects downward from a portion of theperimeter of the radius filler to create a contact area 210 between theperimeter clamp radius filler 112 and a corresponding portion of the topsurface around a perimeter of the base flange 108. The contact area 210defines the boundaries of the relief cavity 212. Pressure is applied tothe base flange 108 by the perimeter clamp radius filler 112 within thecontact area 210 to prevent disbonding at the edges of the stiffenerrunout 102. Because pressure applied to the perimeter clamp radiusfiller 112 is distributed and concentrated around the perimeter of thebase flange 108 while allowing for deformation within the relief cavity212, the perimeter clamp radius fillers 112 provide a significantadvantage over the less effective conventional radius fillers.

According to various embodiments, the contact area 210 may besubstantially C-shaped with an opening 216 that provides access to therelief cavity 212 from one edge of the base flange 108. The opening 216may be positioned along the edge of the base flange 108 opposite the web106 as shown in FIGS. 2 and 3 since that portion of the edge oppositethe web 106 is not highly susceptible to disbonding. The opening 216 mayalternatively be positioned at the most rearward edge or any other edgeof the base flange 108, and may be configured according to any size andshape. According to one embodiment, the relief cavity 212 may be filledwith a compliant filler, such as a sealant or other material appropriateto the particular application, that allows for deformation withoutapplying pressure to the base flange 108. The opening 216 may provide apassage for excess compliant filler to escape from the relief cavity 212during installation.

Fasteners 214 may be threaded or otherwise placed through correspondingapertures in the perimeter clamp radius filler 112, base flange 108, andcomposite structure 104. Torque applied to the fasteners 214 transfersforce from the perimeter clamp radius filler 112 to the perimeter of thebase flange 108 via the contact area 210 to prevent disbonding. Althoughtwo fasteners 214 are shown for each perimeter clamp radius filler 112,any number of fasteners 214 may be used without departing from the scopeof this disclosure.

FIGS. 4 and 5 illustrate two alternative embodiments in which a type ofscarf joint 402 is created between the stiffener runout 102 and thecomposite structure 104. For the purposes of this disclosure, the term“scarf joint” will be used to refer to a coupling between two or morecomponents in which two of the components are manufactured or modifiedto have an angled surface that is engaged with a complementary angledsurface of the other component. FIG. 4 shows a first embodiment in whichthe composite structure 104 has a uniform thickness, but is bonded to afiberglass wedge 404. The fiberglass wedge 404 is hatched in the FIG. 4to highlight the wedge for clarity purposes only. The fiberglass wedge404 provides an angled wedge surface 406 that is co-cured into an angledflange surface 408 of the base flange 108. The angled flange surface 408may be created by reducing the plies of the composite material accordingto the desired angle and joint length. The fiberglass wedge 404 ishighly compliant and is configured to shear load into the compositestructure 104 over a longer length, thereby reducing the local momentsthat may cause disbonding. Although the fiberglass wedge 404 isdescribed herein as being fiberglass, it should be appreciated that anysuitable material that is more compliant (having less stiffness) thanthe material of the stiffener runout 102 and composite structure 104 maybe used to create the wedge.

FIG. 5 shows the second embodiment in which the scarf joint 402 iscreated by reducing the plies of the composite material according to thedesired angle of the angled flange surface 408 and length of the scarfjoint 402. Similarly, the corresponding angled structure surface 502 maybe created by increasing plies of the composite material according tothe desired angle of the angled structure surface 502 and length of thescarf joint 402. The result is a non-uniform thickness of the baseflange 108 that complements a non-uniform thickness of the compositestructure 104 within the scarf joint 402. According to an exampleembodiment, the run to rise ratio associated with the angled structuresurface 502 and angled flange surface 408 of the scarf joint 402 isapproximately 20:1. According to various embodiments, the ratio may bebetween 20:1-30:1. Similar to the effect of a scarf joint 402 createdwith the fiberglass wedge 404 discussed above with respect to FIG. 4,the effect of the scarf joint 402 created by altering the plies of thebase flange 108 and composite structure 104 as shown in FIG. 5 is toshear the loads from the stiffener into the composite structure 104 overa relatively long distance. Doing so minimizes the detrimental localmoments by reducing the offset in the load line that may initiatedisbonding.

Turning now to FIG. 6, an illustrative routine 600 for controllingbondline separation between a stiffener runout 102 and a compositestructure 104 will now be described in detail. It should be appreciatedthat more or fewer operations may be performed than shown in the FIG. 6and described herein. Moreover, these operations may also be performedin a different order than those described herein. The routine 600 beginsat operation 602, where the stiffener runout 102 having a web 106 and abase flange 108 of composite materials is created. It should beappreciated that other features of a stiffener that are not germane tothis disclosure may additionally be created during this or any otheroperation of the routine 600. For example, creation of the stiffener maycommonly include bonding the cap 110 to the top edge of the web 106, aswell as bonding the noodle 114 in the appropriate position between thebottom edge of the web 106 and the first and second flange portions 108Aand 108B.

From operation 602, the routine 600 continues to operation 604, wherethe rounded corners 202 of the base flange 108 are created. Thisoperation may be performed by trimming conventional square corners aftercuring of the base flange 108, or may be a part of the composite moldingprocess. At operation 606, the recessed notch 204 is created in the baseflange 108. The recessed notch 204 may be created via trimming materialfrom the base flange 108 or may be created during the material compositemolding process. The size and depth of the recessed notch 204 may dependon the particular implementation and the desired distance between theflange termination point 206 and the web termination point 208. Theroutine 600 continues from operation 606 to operation 608, where theheight of the web 106 is trimmed from the full height 304 to the reducedheight 306 at the web termination point 208. The leading edge 302 of theweb 106 may be trimmed according to a curved configuration or linearlytapered to the reduced height 306.

From operation 608, the routine 600 continues to operation 610, wherethe scarf joint 402 is created between the stiffener runout 102 and thecomposite structure 104. As discussed above with respect to FIG. 4, afirst method for creating the scarf joint 402 involves utilizing afiberglass wedge 404 bonded to a top surface of the composite structure104, providing an angled wedge surface 406 that is co-cured into anangled flange surface 408 of the base flange 108. The angled flangesurface 408 of the base flange 108 may be created by dropping plies tocreate the desired 20:1-30:1 or other ratio at the scarf joint 402. Alsoas discussed above with respect to FIG. 5, a second method for creatingthe scarf joint 402 includes dropping plies in the base flange 108 tocreate the angled flange surface 408 and increasing plies in thecomposite structure 104 to create the angled structure surface 502. Theangled flange surface 408 and the angled structure surface 502 are thenco-cured to create the scarf joint 402.

The routine 600 continues from operation 610 to operation 612, where theperimeter clamp radius fillers 112 are secured to the base flange 108with fasteners 214. Torque is applied to the fasteners 214 toconcentrate a clamping force within the contact area 210 against theperimeter of the base flange 108 to mitigate disbonding in these areas.At operation 614, the relief cavity 212 is filled with a compliantsealant or other material and the routine 600 ends.

During use, loads from the stiffener runout 102 may be sheared into thecomposite structure 104 at the scarf joint 402 over a relatively longdistance to reduce the offset in the load line that could initiate adisbond between the stiffener runout 102 and the composite structure104. These loads may include a first loading condition in which staticloads are experienced, such as during a steady straight and levelaircraft flight operation. The loads may shift to a second loadingcondition in which dynamic loads are experienced, such as during aclimb, turn, or velocity change. During this load shift, the scarf joint402 and other features of the stiffener runout 102 described above serveto distribute the loads to the composite structure 104 in a manner thatprevents or delays the disbonding that may be present with conventionalstringer runouts.

It should be clear from discussion above that the concepts describedherein may be used independently or in combination to mitigatedisbonding at the stiffener runout 102 under operational loads. Thesubject matter described above is provided by way of illustration onlyand should not be construed as limiting. Various modifications andchanges may be made to the subject matter described herein withoutfollowing the example embodiments and applications illustrated anddescribed, and without departing from the true spirit and scope of thepresent disclosure, which is set forth in the following claims.

What is claimed is:
 1. A stiffener runout, comprising: a web configuredin a substantially vertical orientation and including a front surface, arear surface, a bottom edge, and a web termination point positioned atan outermost location of the web; a base flange, comprising a firstflange portion adjacent to the bottom edge of the web and extendingsubstantially horizontally from the front surface of the web, a secondflange portion adjacent to the bottom edge of the web and extendingsubstantially horizontally from the rear surface of the web, a flangetermination point positioned at an outermost location of the base flangeforward of the web termination point, a recessed notch at the flangetermination point at a central location aligned with the web such thatthe web termination point is positioned substantially at a center of therecessed notch, and a plurality of rounded corners at the flangetermination point; and a perimeter clamp radius filler configured tocontact a top surface of the first flange portion around a portion of aperimeter of the top surface to create a contact area between theperimeter clamp radius filler and the portion of the perimeter of thetop surface, wherein the contact area defines a relief cavity betweenthe perimeter clamp radius filler and the top surface.
 2. The stiffenerrunout of claim 1, wherein the recessed notch comprises an arcuate edgesuch that the web termination point is located on the arcuate edge. 3.The stiffener runout of claim 1, further comprising a compliant fillerdisposed within the relief cavity.
 4. The stiffener runout of claim 1,wherein the contact area is substantially C-shaped such that an openingto the relief cavity is positioned along an edge of the perimeter clampradius filler.
 5. The stiffener runout of claim 4, wherein the openingto the relief cavity is positioned along the edge of the perimeter clampradius filler opposite an edge of the perimeter clamp radius filleradjacent to the web.
 6. The stiffener runout of claim 1, furthercomprising a plurality of fasteners extending through a top surface ofthe perimeter clamp radius filler, through the relief cavity, throughthe first flange portion, and through a structure to which the stiffenerrunout is attached via the plurality of fasteners.
 7. A stiffener runoutassembly, comprising: a stiffener runout, comprising a web trimmed froma full height at a location prior to a web termination point to areduced height at the web termination point at an outermost location ofthe web, a base flange extending horizontally in opposite directionsfrom the web and including a flange termination point positioned at anoutermost location of the base flange, wherein the flange terminationpoint is positioned forward of the web termination point, and whereinthe web termination point is positioned within a recessed notch of thebase flange; a composite structure bonded to the base flange of thestiffener runout at a scarf joint; a pair of perimeter clamp radiusfillers positioned on a top surface of the base flange on opposing sidesof the web, each perimeter clamp radius filler comprising a bottomsurface configured to contact the top surface of the base flange arounda portion of a perimeter of the top surface to create a contact areabetween the perimeter clamp radius filler and the portion of theperimeter of the top surface, wherein the contact area defines a reliefcavity between the perimeter clamp radius filler and the top surface;and a plurality of fasteners securing the pair of perimeter clamp radiusfillers to the top surface of the base flange such that each fastenerextends through a top surface of one of the pair of perimeter clampradius fillers, through the relief cavity, through the base flange, andthrough the composite structure.
 8. The stiffener runout assembly ofclaim 7, further comprising a fiberglass wedge positioned between thebase flange and the composite structure, wherein the base flangecomprises a non-uniform thickness corresponding to a configuration ofthe fiberglass wedge to create the scarf joint.